Add informative error message for known point of failure (observers like Tap coverage on Node 8)

[AlanSl]

0x fails in lib/ticks-to-tree.js if it is running inside a Tap test with coverage enabled on Node 8, which causes lib/random-require.js to return an empty string. There are probably other similar scenarios too, where something observing child processes interferes.

The point it fails doesn't give much of a clue about what happened - it's an assert in a file two removed from where the problem actually occurs, and the problem itself happens in random-require.js which is hard to debug because it uses console.log output to communicate with the parent process that calls it. Then quite a leap of logic is needed to link this to the actual cause (e.g. Tap coverage).

So it's helpful to give users and contributors an informative error message at this point.

[mcollina]

LGTM